The limited digestion of supercoiled DNA of known primary structure by single-strand specific nucleases will be investigated. Mung bean nuclease and venom phosphodiesterase will be used to probe 0X174 RF for sites with single-strand character. Through the use of restriction enzymes, gel electrophoresis, and 32P-end labeling, the cleavage sites will be located within the known sequence of 0X174. Linear, duplex DNAs which contain only one nuclease susceptible site will also be used as substrates for mung bean nuclease. DNA fragments of known primary structure will be obtained by restriction enzyme digestion of 0X174 RF. The sites cleaved by the single-strand specific enzyme will be localized as in the studies with supercoiled DNA. Block polymers of the general formula d(CnAmCn).d(GnTmGn), where n equals 2, 4 or 6 and m equals 5-10 will be synthesized using enzymatic methods. Investigation of the effects of solution conditions on both the nuclease susceptibility of the effects of solution conditions on both the nuclease susceptibility and the physical properties of these block polymers should help to define the meaning of the term "single-strand character". The effects of single-strand specific nucleases on chromatin will be examined using gel electrophoretic techniques. These studies may reveal the structural alterations of DNA caused by the formation of nucleosomes or higher order nucleoprotein complexes.